1. Field of the Invention
The present invention relates to a lithium polymer battery which is simply fabricated, and which has an improved assembly structure in which a positive electrode, a negative electrode and a separator are stacked, and a manufacturing method thereof.
2. Description of the Related Art
Lithium secondary batteries are classified into lithium metal batteries, lithium ion batteries and lithium polymer batteries according to the type of lithium used. A lithium polymer battery is a battery in which lithium ions move between a cathode made of lithium oxide and a negative electrode made of a carbon material during charging/discharging to generate an electromotive force. The lithium polymer battery has an advantage in that it is explosion-free due to the use of a solid polymer electrolyte having an ion conductivity higher than a liquid electrolyte. Also, unlike other kinds of batteries, the lithium polymer battery can overcome the problem of battery performance deteriorating due to byproducts generated during charging and/or discharging. Furthermore, due to bendability, much attention has recently been paid to the lithium polymer batteries as next generation batteries.
As shown in FIG. 1, a conventional lithium polymer battery 10 includes a positive electrode (or a cathode in a battery) 11 consisting of a positive electrode current collector and a positive electrode active material layer tightly fixed to at least one surface of the positive electrode current collector, a negative electrode (or an anode in a battery) 12 consisting of a negative electrode current collector and a negative electrode active material layer tightly fixed to at least one surface of the negative electrode current collector, and a separator 13 interposed between the positive electrode 11 and the negative electrode 12, for insulating the positive electrode 11 and the negative electrode 12 from each other. The positive electrode 11, the negative electrode 12 and the separator 13 constitute an assembled battery and a plurality of assembled batteries are stacked to form the battery 10.
As shown in the drawing, a positive electrode tab 14 and a negative electrode tab 15 are drawn out from the positive electrode 11 and the negative electrode 12, respectively, along the peripheries of each electrode plate. A plurality of positive electrode tabs 14 and a plurality of negative electrode tabs 15 are collected and are bent into a V-shape. The ends of the V-shaped positive and negative electrode tabs 14 and 15 are welded to a positive electrode terminal 16 and a negative electrode terminal 17, respectively.
The battery 10 having the above-described configuration is mounted within a case 18. The case 18 has a space 18a in which the battery 10 is accommodated, and the positive electrode terminal 16 and the negative electrode terminal 17 protrude to the outside of the case 18 when the case 18 is hermetically sealed.
Here, various types of the lithium polymer battery 10 can be fabricated. In particular, assembled batteries which have recently been put into practical use include bi-cell assembled batteries. A bi-cell assembled battery is constructed such that a separator is fixed to both sides of a negative electrode and a positive electrode is fixed to the outer surface of the separator.
The lithium polymer battery 10 shown in FIG. 1 is constructed such that a plurality of bi-cell assembled batteries are stacked and the positive and negative electrode tabs 14 and 15 are drawn out therefrom and welded to the positive electrode terminal 16 and the negative electrode terminal 17, respectively.
The conventional lithium polymer battery 10 is fabricated as follows. First, a plurality of positive and negative electrode current collectors are provided. As described above, a positive electrode active material layer and a negative electrode active material layer are tightly fixed to both surfaces of the positive and negative electrode current collectors, respectively, thereby completing the positive electrode 11 and the negative electrode 12. Then, the stacked plurality of positive and negative electrode tabs 14 and 15 drawn out from the positive and negative electrode current collectors are collected, respectively, and the battery 10 is accommodated within the case 18.
Next, the ends of the collected positive and negative electrode tabs 14 and 15 are welded to the positive and negative electrode terminals 16 and 17, respectively.
During the above-described procedure, some of the plurality of positive and negative electrode tabs 14 and 15 may not be properly connected to the positive and negative electrode terminals 16 and 17. Accordingly, the internal resistance of the battery 10 may increase, which adversely affects the performance of the lithium polymer battery 10.
The positive electrode 11 and the negative electrode 12 have positive and negative electrode active material layers on both surfaces of positive and negative electrode current collectors. Since the positive electrode active material layer on the rear surface of the positive electrode 11 is relatively far from the negative electrode 12 compared to the positive electrode active material layer on the front surface of the positive electrode 11, movement of lithium ions does not occur smoothly, which lowers high-rate discharging efficiency.
Since several steps of lamination are necessary for fabricating an assembled battery having the positive electrode 11, the negative electrode 12 and the separator 13 interposed therebetween, and a process of stacking a plurality of bi-cell assembled batteries is necessary for completing the lithium polymer battery 10, there are limits associated with continuous mass production. Also, burs may be generated at the ends of positive and negative electrode current collectors in the course of cutting the positive electrode 11, the negative electrode 12 and the separator 13 to a desirable size. Burs may short-circuits in a battery and undesirably increase the rate of defectiveness in completed batteries.
To solve the above problems, it is an objective of the present invention to provide an improved lithium polymer battery in view of the structure and manufacturing method thereof, in which an assembled battery is fabricated by winding a positive electrode, a negative electrode and a separator to then be mounted in a case, and a method for manufacturing the same.
Accordingly, to achieve the above objective, there is provided a lithium polymer battery having a positive electrode consisting of a positive electrode current collector and positive electrode sheets formed on at least one surface of the positive electrode current collector, the positive electrode sheets having a positive electrode active material layer as a main component, a negative electrode consisting of a negative electrode current collector and negative electrode sheets formed on at least one surface of the negative electrode current collector, the negative electrode sheets having a negative electrode active material layer as a main component, and a separator interposed between the positive electrode and the negative electrode, for insulating the electrodes from each other, wherein the positive electrode and the negative electrode of the lithium polymer battery are each formed of a plate and are wound with the separator interposed therebetween.
Also, the positive and negative electrode current collectors are preferably formed of either an expanded metal or a punched metal.
Further, the separator is preferably formed of polyethylene.
According to another aspect of the present invention, there is provided a method for manufacturing a lithium polymer battery, the method including the steps of preparing active material slurry for positive and negative electrode sheets, preparing the positive and negative electrode sheets using the active material slurry, preparing a positive electrode and a negative electrode by tightly fixing the positive and negative electrode sheets to at least each one surface of positive and negative electrode current collectors, extracting a plasticizer from the positive and negative electrodes, positioning a separator between the positive and negative electrodes and winding the resultant structure to complete a battery unit, mounting the battery within a case and impregnating a predetermined amount of an electrolytic solution, and hermetically sealing the case to complete the battery.
Also, in the step of completing the battery unit, the cross section of the battery unit is preferably made elliptic by applying a unidirectional tensile force so as to compactly mount the battery unit in a space of the case when simultaneously winding the positive electrode, the separator and the negative electrode.